This invention relates to composite asymmetrical membranes which are manufactured from polymers in such a way that an extremely thin film is applied as filtration membrane onto a porous supporting membrane.
For some years, the membrane filtration, which is well-known as ultrafiltration and reversed osmosis, has become more and more important for the selective separation of fluid particles from a liquid containing a plurality of components, for example a solution.
When a solution is brought to the surface of the filtration membrane, the solvent and a part of the dissolved substances in the solution are retained at the surface of the membrane. The filtration of solutions of macromolecular substances in which the high molecular particles are retained is called "ultrafiltration," while the filtration of solutions of low molecular substances in which the low molecular particles are retained is called "reversed osmosis."
In the field of sea water and brackish water desalting, for the removal of contaminations from water and for other purposes, the reversed osmosis has more and more proved to be a simple and economical process. The degree of efficiency of the reversed osmosis decisively depends on the filtration efficiency and the retaining capacity of the membranes used, but also on their stability and durability.
Up to now, so-called integral-asymmetrical membranes have been used almost exclusively, which membranes are manufactured according to a method used for the first time by S. Loeb and S. Sourirajan (Advan. Chem. Ser. 38, 117 (1963)) by precipitation from a solution containing the polymer membrane material. An extremely thin polymer film is thus formed on a highly porous substructure which serves as supporting material for the film having a thickness of approx. 1/um. As the filtration efficiency is inversely proportional to the film thickness, attempts have been made to prepare films that are even thinner. Hence, there have been developed composite asymmetrical membranes, so-called "composite membranes," which are manufactured in such a way that an extremely thin film is applied as filtration membrane onto a porous supporting membrane only subsequently. These composite membranes were described by Riley, R.L., H.K. Lonsdale, L.D. LaGrange and C.R. Lyons in Office of Saline Water R. & D. Report No. 386, 1968. Such films have a thickness of about 500 A only and, therefore, they have a considerably higher filtrate flow than the comparatively thick integral-asymmetrical membranes, the retaining capacity being the same.
Said process for the manufacture of composite membranes, however, is limited to the use of such materials the solvent of which does not attack the porous supporting membrane. It is necessary either to find a supporting membrane that is not dissolved by the solvent in which the material of the filtration membrane is dissolved, or -- with a given supporting membrane -- only such polymers can be used for the filtration membrane the solvent of which does not influence the supporting membrane. As a result, the number of starting materials, in particular the number of polymers, having a high chemical and thermal stability is considerably reduced. The operational possibilities of the composite membranes are reduced to such conditions, for example pH-value and temperature, that are compatible with the material of the filtration membrane and of the supporting membrane. The diverse mechanical properties of filtration membrane and supporting membrane may also have a detrimental effect. When masking the supporting membrane with a substance which is insoluble in the solvent of the filtration membrane but is eliminated with water after the filtration membrane has been applied (cf., inter alia, H.K. Lonsdale, R.L. Riley, L.D. LaGrange, C.R. Lyons, A.S. Douglas, U. Merten, Office of Saline Water R. & D. Report No. 484, 1969), it is necessary to find substances that are soluble in water but insoluble in the solvent of the filtration membrane, whereby the number of polymers suitable for the filtration membrane is again reduced.
Commonly-owned and co-pending U.S. Pat. application Ser. No. 458,483 now U.S. Pat. No. 3,925,211 discloses a two-stage-reaction, membrane-forming process which applicants have utilized in forming novel composite membranes as discussed below.